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Do stellar magnetic cycles influence the measurement of precise radial velocities?

N. C. Santos, J. Gomes da Silva, C. Lovis, C. Melo

Abstract
The ever increasing level of precision achieved by present and future radial-velocity instruments is opening the way to the discovery of very low mass, long period planets (e.g. solar-system analogs). These systems will be detectable as low amplitude signals in radial-velocity (RV). However, an important obstacle to their detection may be the existence of stellar magnetic cycles with similar timescales. Here we present the results of a long term program to measure simultaneously radial-velocities and stellar activity indicators (CaII, Hα, HeI) for a sample of stars with known activity cycles. Our results suggest that all these stellar activity indexes can be used to trace the stellar magnetic cycle in solar-type stars. Further to this, we find clear indications that different parameters of the HARPS cross-correlation function (BIS, FWHM, and Contrast) are also sensitive to activity level variations. Finally, we show that though in a few cases slight correlations or anti-correlations between radialvelocity and the activity level of the star exist, their origin is still not clear. We can however conclude that for our targets (early-K dwarfs) we do not find evidence for radial-velocity variations induced by variations of the stellar magnetic cycle with amplitudes significantly above ∼1m/s.

Keywords
planetary systems – Stars: activity – Stars: fundamental parameters – Techniques: spectroscopic – Techniques: radial velocities – starspots

Notes
Based on observations collected at the La Silla Parana Observatory, ESO (Chile) with the HARPS spectrograph at the 3.6-m telescope (ESO runs ID 072.C-0096 073.D-0038 074.D-0131 075.D-0194 076.D-0130 078.D-0071 079.D-0075 080.D-0086 081.D-0065).
Tables 5 to 8, with the data used for Figs. 2, 3, and 8, are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/511/A54

Astronomy and Astrophysics
Volume 511, Number of pages A54_1
2010 February

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia